Retinoids are natural and synthetic derivatives of vitamin A (retinol), which modulate different cellular processes, including proliferation, differentiation and apoptosis. They are compounds of clinical interest for the treatment and prevention of a variety of tumors. Fenretinide, or N-(4-hydroxyphenyl)retinamide (4-HPR), is a synthetic retinoid (an amide of all-trans retinoic acid), which, in preclinical models, proved to be less toxic than many other retinoids while maintaining a significant biological activity. (Moon et al., Cancer Res., 1979, 39:1339–1346). In animal models, 4-HPR had preventive efficacy in mammary, prostate and colon carcinogenesis and lymphomagenesis and therapeutic efficacy against neuroblastoma, mammary, ovary and Kaposi's tumors (Formelli et al., FASEB J. 1996, 10:1014–1024; Ferrari et al., Clin. Cancer Res. 2003, 9:6020–6029). In clinical trials in adults (Veronesi et al., J. Natl. Cancer Inst. 1999, 91:1847–56) and in children (Garaventa et al., Clin. Cancer Res. 2003, 9:2032–2039), 4-HPR had a favorable toxicity profile and was better tolerated than other retinoids. Its main side effect was impaired dark adaptation, which was due to reduction of retinol plasma levels (Formelli et al., Cancer Res. 1989, 49:6149–6152). In humans, 4-HPR has shown efficacy in premalignant lesions such as oral leucoplakia (Chiesa et al., Oral Oncol. Europ. J. Cancer, 1992, 28B(2):97–102), lichen planus (Tradati at al., Cancer Lett., 1994, 76:109–111) and actinic keratoses (Moglia et al., Cancer Lett. 1996, 110:87–91) and has shown promising results for the prevention of breast and ovarian tumors (Veronesi et al., J. Natl. Cancer Inst. 1999, 91:1847–56; De Palo et al., Gynecol. Oncol., 2002, 86:24–27). In a clinical trial in women treated with 4-HPR after having undergone surgery for early breast cancer, 4-HPR reduced the incidence of new breast cancer in premenopausal women (Veronesi et al., J. Natl. Cancer Inst. 1999, 91:1847–56). In the same trial, a significant reduction in the incidence of ovarian cancer was also observed during the treatment period (De Palo et al., Gynecol. Oncol., 2002, 86:24–27). Actually, the retinoid is under investigation as a preventive and therapeutic agent for neuroblastoma, breast and ovarian tumors.
The tumor preventive and therapeutic activity of 4-HPR has been attributed to its potent growth inhibitory effects, associated with induction of apoptosis, which have been shown in tumor cells of different histiotypes, including breast, prostate, ovary, head and neck, neuroblastoma and leukemia (Formelli et al., FASEB J. 1996, 10:1014–1024). Considerable attention has been directed towards understanding the mechanism of action of this retinoid and different molecular targets have been implicated in its growth inhibitory activity.
An important aspect that may contribute to the biological effects of 4-HPR is its metabolism. It is known that 4-HPR is extensively metabolized in vivo (Hultin et al.,. Drug Metab. Dispos., 1986, 14:714–717; Formelli et al., Cancer Res., 1989, 49:6149–6152; Mehta et al., Eur. J. Cancer, 1991, 27:138–141), but it is not known whether 4-HPR or one of its metabolites is the active agent and how they interact with each other. Moreover, nothing is known about 4-HPR metabolism in tumor cells. To date, the only identified metabolite of 4-HPR is N-(4-methoxyphenyl)retinamide (4-MPR), which is less polar than the parent drug and which has been found in all the in vivo studies in mice, rats and humans. 4-MPR does not seem to play a role in 4-HPR tumor growth inhibitory effect because it was ineffective in inhibiting proliferation in different tumor cell lines (Appierto et al., Br. J. Cancer., 2001, 84:1528–1534; Metha et al., Eur. J. Cancer, 1998, 34:902–907). Moreover, in mice bearing a human ovarian carcinoma, 4-MPR had no antitumor effect and it did not increase 4-HPR activity (Formelli et al., Eur. J. Cancer, 2000, 36:2411–2419). In addition to 4-MPR, all the indicated studies also reported the presence of unidentified metabolites more polar than the parent drug. In one of these studies, it was found that in normal and tumor breast tissues of women treated with 4-HPR, 4-MPR was mainly localized in the fat tissue, whereas 4-HPR and an unidentified polar metabolite were concentrated in epithelial cells, that is in the cells where the drug is supposed to exert its effect (Metha et al., Eur. J. Cancer, 1991, 27: 138–141).